1. Field of the Invention
The present invention relates to plasma ignition devices for use in ignition of internal combustion engines and, more particularly, relates to an ignition device for an internal combustion engine equipped with a power circuit which can charge a PJ capacitor to a target charging voltage within a predetermined time even in the case where a charging time period of the PJ capacitor is short and a capacitance value thereof is large.
2. Description of the Related Art
A known plasma ignition system, which ejects plasma jets in a compressed air-fuel mixture, includes a tank capacitor which is for charging a plasma jet (hereinafter, only referred to abbreviated as “PJ”) capacitor and a current limiting resistor in a power circuit of a plasma ignition device and gives large ignition energy to the compressed air-fuel mixture in the case of ignition to improve ignition quality. (For example, see Patent Document 1.)
[Patent Document 1] Japanese Translation of PCT International Application No. 2000-511263
In the foregoing plasma ignition device, there is a problem in that a charging time period of the PJ capacitor in the power circuit is short during engine high rotation and, more particularly, the PJ capacitor cannot be charged to the target charging voltage in the case where a capacitance value of the PJ capacitor is large; and therefore, required plasma energy cannot be satisfied.
FIG. 10 shows a circuit diagram of the known plasma ignition device disclosed in the aforementioned Patent Document 1; FIG. 11 shows its timing chart; and the foregoing problem will be described based on the principle of operation.
In FIG. 10 and FIG. 11, when battery power 1 is supplied at time t1, a DC/DC converter 2 in a power circuit 100 starts to operate and charges a tank capacitor 5 and a PJ capacitor 9.
When a charging voltage VC2 of the tank capacitor 5 reaches a voltage limiting value VCL2 of a voltage limiting circuit 3 at time t2, the operation of the DC/DC converter 2 is made to stop.
A high voltage V2 is applied to a spark plug 20 at time t3; accordingly, a dielectric breakdown is generated between electrodes, plasma energy is given from the power circuit 100 to discharge space where impedance is lowered due to starting of discharge, and plasma is ejected; and therefore, a plasma current PJ-I1 flows. The plasma current PJ-I1 flows; and accordingly, electric charge charged in the PJ capacitor 9 is discharged and a charging voltage VC1 becomes 0 V.
After that, when an operation mode is switched to a high rotation mode at time t4, as described above, the plasma current PJ-I1 flows; and accordingly, the electric charge charged in the PJ capacitor 9 is discharged and the charging voltage VC1 becomes 0 V at time t5. After that, the tank capacitor 5 and the PJ capacitor 9 are charged at time t5 to t6; however, an ignition cycle becomes short because of the high rotation mode; that is, the charging time periods of the tank capacitor 5 and the PJ capacitor 9 become short, and the charging voltage VC2 of the tank capacitor 5 cannot reach the voltage limiting value VCL2 of the voltage limiting circuit 3 at time t6; and accordingly, the charging voltage VC1 of the PJ capacitor 9 cannot also be charged to a target charging voltage VC1max. Accordingly, there is a problem in that even when the high voltage V2 is applied to the spark plug 20 at time t6, the dielectric breakdown is generated, and the plasma energy is given from the power circuit 100 to the discharge space where the impedance is lowered due to starting of discharge, the plasma energy in the case of ejecting plasma becomes low with respect to target plasma energy.